Many modern electronic circuits, such as those used in communications, use a high tolerance, low noise reference frequency source. These reference frequency sources typically contain quartz crystal oscillator circuits. Active devices having gain are used to excite the crystal oscillator, which takes a finite amount of time to reach steady state operation from start-up. Optimum low noise performance from a crystal oscillator is dependent on the gain of the active devices. This gain contributes to the noise of the oscillator. The gain to achieve the desired oscillation amplitude is dependent on the crystal resistance of the crystal oscillator.
However, the crystal resistance is not constant, typically being higher at start-up than when oscillating in steady state. The crystal resistance is related to the Q factor of the oscillator, which determines the amount of power applied to the crystal to keep it oscillating at the same amplitude. As the resistance decreases, the amount of power consumed decreases. The variation in the crystal resistance causes more power to be used at start-up than is desired to achieve the best noise performance in steady state operation. However, decreasing the power such that optimal noise performance is achieved in steady state increases the amount of time for the crystal oscillator to reach steady state from start-up. Conversely, it is desirable in many applications for the crystal oscillator to reach a stable steady state frequency in a minimum amount of time. Accordingly, it is desirable to provide a reference oscillator that has both a fast start-up time and low noise at steady state operation.
In addition, physical variations in the crystals themselves cause the amplitude of the crystal oscillations to vary. Thus, the crystals in a batch of crystals may have different steady state oscillation frequencies over a particular amplitude range. Similarly, variations in the ambient temperature of the oscillator circuit may cause the steady state oscillation amplitude of a particular crystal to fluctuate due to changes in the gain of the circuit. It is thus additionally desirable to provide an arrangement that provides compensation for both crystal-to-crystal variation and temperature variation for a single crystal.
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